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Stratigraphic range: 566–555 Ma
The Longmyndian Supergroup is a 6,000-metre-thick (20,000 ft) sequence of Late Precambrian rocks that outcrop between the Pontesford–Linley Fault System and the Church Stretton Fault System in the Welsh Borderland Fault System. The supergroup consists of two major geological groups, the Stretton Group and the overlying Wentnor Group. The rocks are a generally regressive sequence from basinal facies to clastic sedimentation. The rocks are thought to be derived from Uriconian mountains that were formed during the southward subduction of an oceanic plate beneath a continental block (ocean closure). The rocks have since been folded due to fault movements and plunge gently to the south.
The Longmyndian rocks were deposited in northeast–southwest trending faulted rift basins. These were deposited on top of the Uriconian volcaniclastic deposits. Subsequent ocean closure squeezed these rocks together to provide sub-vertical bedding in the synclinal sequence we see in exposed rocks today. As noted below, the deposited rocks of the Longmyndian show a variety of depositional facies that tell a story of ocean closure. The Longmyndian deposits rest upon Uriconian rocks but different sections are exposed exclusively between the Church Stretton Fault and the Pontesford-Linley Lineament.
The Wentnor Group is made up of the fluvial deposits of the Bridges Formation and is the topmost of the supergroup. This is underlain by the braided deposits of the Bayston-Oakwood Formation which completes the Wentnor Group.
The topmost of the Stretton Group, the Portway Formation, consists of braided fluvial deposits with marine incursions. The underlying Lightspout Formation is interpreted as a fluvio-deltaic environment, this is succeeded by a similar facies in the Synalds Formation. Turbidite facies are prevalent in the Burway Formation and the deep marine basinal environments continue toward the base of the supergroup within the Stretton Shale Formation. The Ragleth Tuff Formation completes the Stretton Group and the supergroup sequence.
Within the strata are bentonite and lapilli tuff horizons that allow for zircon U-Pb dating and noted within the Batch Volcanics of the Synalds Formation, toward the base of the Stretton Group, a date is returned of 566±2.9 Ma. The Lightspout Formation toward the top of the Stretton Group returned a date of 555.9±3.5 Ma.
- Brenchley, P. J.; Rawson, P. F. (2006). The Geology of England and Wales (2nd ed.).[ISBN missing]
- Pauley, J. C. (1991). "A revision of the stratigraphy of the Longmyndian Supergroup, Welsh Borderland, and its relationship to the Uriconian Volcanic Complex". Geological Journal. 26 (2): 167–183. doi:10.1002/gj.3350260209.
- Compston, W.; Wright, A. E.; Toghill, P. (2002). "Dating the Late Precambrian volcanicity of England and Wales". Journal of the Geological Society. 159 (3): 323–339. doi:10.1144/0016-764901-010.
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